Diffusion Current Density Research Articles

Electrolyte concentration field in a continuous flow electrolytic reactor

Abstract An analytical solution to the problem of the electrolyte concentration field in a continuous flow electrolytic reactor is presented for the special case of flat velocity profile, negligible lateral ionic diffusion and constant electrode current densities. The approach is illustrated by a practical example.

Diffusion along Small-Angle Grain Boundaries in Silicon

Diffusion fronts in samples containing grain boundaries are spike shaped. Velocity of spike advance and angle between spike and boundary are measured. The "spike-velocity method" of analysis permits evaluation of two effective widths, ${W}_{D}$ and ${W}_{0}$, which describe the diffusion properties of the boundary. This method has been used to analyze data on phosphorus diffusion into boron-doped silicon at 1200\ifmmode^\circ\else\textdegree\fi{}C and 1050\ifmmode^\circ\else\textdegree\fi{}C. It is concluded that an enhanced diffusion current flows along each dislocation of the grain boundary over a cross section less than one Burgers-vector square. The diffusion current density is about 300 000 times that of the bulk. This corresponds to an energy of 1.5 ev by which grain boundary diffusion is favored over the bulk diffusion. This enhancement is believed to be caused by enrichment of phosphorus and also partly by the extra concentration of vacancies near the dislocation cores. Some possible extensions of the studies to include saturation effects at the dislocation cores are discussed.

Critical p H and Critical Current Density for Passivity in Metals

Based on electrochemical mechanisms, an equation is derived for the relation of to the critical current density necessary to passivate metals: where and λ are constants. Accordingly, a linear relation beween and is observed for Cr, Ni and 14% Cr‐Fe alloys. When passivity occurs by reason of dissolved oxygen, it is shown that usually equals the limiting diffusion current density for O2. A critical exists above which passivity is stable, but not below. This critical pH decreases linearly with the logarithm of dissolved O2 concentration. Calculated and observed values for the critical of 14% Cr‐Fe alloy are in reasonable agreement.In absence of O2, passivity is achieved when the rate of reduction of H+ exceeds . Metals in this category must have Flade potentials more active than the hydrogen electrode in the same solution. More active Flade potentials accompany greater thermodynamic stability of the passive film, accounting in part for greater resistance to breakdown of passivity by specific anions. Among the metals in this category achieving high corrosion resistance in deaerated acids are Ti, Ta, Zr, and Mo.